Ink jet printers are well known in the art. Generally, an ink jet direct printer includes a print head having an array of nozzles or orifices, a supply of ink and a plurality of ejection elements (typically either expanding vapor bubble elements or piezoelectric transducer elements) corresponding to the array of nozzles for ejecting the ink from the nozzles. The ink ejected in this manner forms drops which travel along a trajectory or flight path until they reach a print medium such as a sheet of paper, overhead transparency, envelope or the like. Once they reach the print medium, the drops dry and collectively form a print image. Typically, the ejection elements are selectively energized so that a predetermined or desired print image is achieved.
In some applications, it is desirable to employ an ink jet transfer printer instead of an ink jet direct printer. The ink jet transfer printer is similar to the ink jet direct printer except that the ink jet transfer printer does not print directly on the print medium. Instead, the ink jet transfer printer prints a negative of a desired image onto an intermediate print surface. The print surface is subsequently brought into contact with the print medium so that the image from the print surface transfers to the print medium. Thus, it is apparent that the maintenance of the print surface and the interaction of the ink with the print surface significantly influence print quality. For example, if the ink "wets" too much (spreads too thinly) upon contact with the print surface, then a distorted image will likely result. On the other hand, if the ink "beads" too much (forms a drop with little surface contact with the print surface per unit volume of ink), then a distorted image will also likely result.
Recently, the postage meter industry and other envelope printing industries have begun to incorporate ink jet printers. A typical postage meter applies evidence of postage, commonly referred to as a postage indicia, to an envelope or other mailpiece and accounts for the value of the postage dispensed. In this manner the dispensing of postal funds is accurately tracked and recorded.
In traditional postage meters, two types of printing means are employed: one being a die plate located on the peripheral surface of a print drum that is adapted to print the fixed portion of the postage indicia, such as the graphics design, town and state, while the other print means is adapted to print the variable portion of the postage indicia, such as the date and value of postage dispensed. In such traditional postage meters, this printing means usually includes a plurality of print wheels which project through suitable apertures formed in the curved surface of the die plate. Each print wheel contains a plurality of alpha-numeric characters which are selectively rotatable to project through the die plate. In order to print the postage indicia, the print drum is rotated and the die plate and the print wheels are suitably inked prior to the print drum coming into contact with the envelope. After inking, the print drum continues to rotate and the die plate and the print wheels are brought into contact with the envelope where the ink transfers to the envelope resulting in the postage indicia printed on the envelope.
In order to vary the postage value and the date, the operative positions of the print wheels, which are mounted to bodily rotate with the interior of the print drum, must be changed. Since these print wheels are not directly accessible to the operator, the print wheels are operatively connected to an associated setting mechanism which is also located primarily inside the print drum. The setting mechanism is normally connected to a motor for rotating the print wheels to a desired position in response to inputs from a keyboard. In the alternative, the setting mechanism is connected to levers which extend outside the postage meter housing so that the operator can manipulate the print wheels manually. Both of these arrangements necessitate a rather intricate, complex and costly mechanism to enable the print wheels to be set to a desired position and then rotated along with the print drum through a print cycle.
The postage meters disclosed in U.S. Pat. No. 3,869,986 entitled INK JET PRINTING POSTAGE PRINTING APPARATUS and U.S. Pat. No. 4,673,303 entitled OFFSET INK JET POSTAGE PRINTING, both of which are assigned to the assignee of the present invention, depart from the traditional postage meters described above by incorporating ink jet printing technology. The print wheels and associated setting mechanisms are dispensed with and replaced with an ink jet print head.
Ink jet printers are well known in the art. Generally, an ink jet printer includes an array of nozzles or orifices, a supply of ink and a plurality of ejection elements (typically either expanding vapor bubble elements or piezoelectric transducer elements) corresponding to the array of nozzles for ejecting the ink from the nozzles. The ink ejected in this manner forms drops which travel along a trajectory or flight path until they reach a print medium such as a sheet of paper, overhead transparency, envelope or the like. Once they reach the print medium, the drops dry and collectively form a print image. Typically, the ejection elements are selectively energized so that a predetermined or desired print image is achieved.
In U.S. Pat. No. 3,869,986, the ink jet print head is positioned to print directly on the envelope through suitable apertures in the print drum while the print drum is rotating through a print cycle. The print drum still contains a die plate for printing the fixed portion of the postage indicia while the ink jet print head prints the variable portion.
Although this system removes the complexity of the setting mechanism, it suffers from various drawbacks. For example, the nozzles of the print head must be energized at the precise moment when the aperture appears between the print head and the envelope. Otherwise, the ink will deposit on the inner surface of the print drum instead of the envelope. Thus, the timing of the rotation of the print drum and the energizing of the nozzles must be precisely controlled. As another example, no provisions are made for the maintenance of the print head. Left unattended, the print head may either drip ink or in the alternative become clogged due to evaporation of ink or an accumulation of paper dust and other contaminants. Clearly, either event is undesirable. Another drawback is that the print head is spaced far away from the envelope due to the thickness of the print drum. Thus, a large print gap is created which reduces print quality.
In U.S. Pat. No. 4,673,303, the ink jet print head is positioned to print on the print drum. The print drum includes a first region containing the die plate of the fixed portion of the postage indicia and a second region which receives ink from the print head. During a print cycle, the second region is depressed below the periphery of the print drum so that the inking rollers for the first region do not spread ink onto the second region. Then, the second region is brought back into alignment with the periphery of the print drum and the print head applies ink thereon to form the variable portion of the postage indicia. As the print drum comes into contact with the envelope, both the ink from the die plate and the second region are transferred to the envelope.
Although this system removes the complexity of the setting mechanism, it suffers from various drawbacks. For example, depressing and raising the second region during a print cycle requires a complex mechanism. Additionally, this system does not provide for maintenance of the print head. Therefore, it suffers from the same drawbacks as previously discussed above. As another example, this system makes no provisions for maintenance of the second region. Therefore, stray ink drops could collect on the second region and transfer to the envelope during printing which would result in a poor quality postage indicia. Still another example, residual ink which did not transfer to the envelope would remain on the second region and contaminate subsequent printing.
For all of the above reasons, it becomes apparent that there are difficulties and drawbacks associated with the prior art postage meters employing ink jet printers. Therefore, there is a need for an ink jet transfer printer which substantially overcomes the problems associated with the prior art.